1. Field of the Invention
The present invention is related to the stabilization of aromatic diamines. More particularly, the present invention is related to the use of a hindered amine light stabilizer to reduce the discoloration of aromatic diamines at high temperature.
2. Description of Related Art
The use of stabilizers in polyurethane elastomers is well known in the art. These stabilizers are often used to protect the polyurethane elastomer or coating from the effects of heat, light, and ultraviolet radiation. Primary antioxidants, such as hindered phenolics, are often used to protect against oxidation at elevated temperatures. UV absorbers, such as the benzotriazoles, and hindered amine light stabilizers (HALS) can be used to retard discoloration (yellowing) of cured polyurethanes.
In the preparation of polyurethane elastomers, aromatic diamines that are solid at room temperature are often used as a curative. These compounds need to be melted at elevated temperatures in order to be used. The most common of these high melting diamines is methylene bis-orthochloroaniline (MOCA). Commercial, technical grade, MOCA is actually about 90% methylene bis-orthochloroaniline and 7-8% of a triamine analog containing three aromatic rings instead of two. MOCA is commonly heated to about 130xc2x0 C. and held as a liquid for use in polyurethane elastomer production operations.
Maintaining these aromatic diamines as liquids at high temperature can present problems for the polyurethane elastomer producer. Over time, the curative begins to degrade. The first problem this causes is that the color becomes darker, which changes the color of the elastomer produced. Thus, the elastomer producer is often required to discard the curative as a toxic waste and begin with fresh material. If further degradation is allowed, the properties of the polyurethane elastomer can also suffer, which often leads to failure of the elastomer in use (such as in an industrial tire).
Thus, it is desirable to develop a method for the stabilization of aromatic diamines, particularly MOCA, against the effects of high temperature degradation. Preferably, the stabilizer would prevent both the discoloration of the aromatic diamine and the formation of byproducts and associated loss of assay.
U.S. Pat. No. 3,640,928 discloses the stabilization of synthetic polymers including a polyolefin, a polyvinyl chloride, a polyvinylidene chloride, a polyurethane, and a polyamide against photo- and thermo-deteriorations thereof by incorporating therein a piperidine derivative in a sufficient amount to prevent such deteriorations thereof, usually in an amount of 0.01 to 5.0% by weight based on the amount of the synthetic polymer.
U.S. Pat. No. 3,840,494 discloses the stabilization of synthetic polymers including a polyolefin, a polyvinyl chloride, a polyvinylidene chloride, a polyacetal, a polyester, and a polyamide against photo- and thermo-deterioration by incorporating therein monobasic, dibasic, tribasic, or tetrabasic acid esters of 4-piperidinol derivatives in an amount sufficient to prevent such deterioration.
U.S. Pat. Nos. 4,021,432 and 4,046,737 and U.K. Patent No. 1,399,239 disclose 1- and 4-substituted piperidines as stabilizers for organic material. They are produced by reacting the corresponding 1-substituted piperidinols with acid chlorides or corresponding 4-substituted piperidines with a compound introducing a residue into the 1-position.
U.S. Pat. No. 5,874,619 discloses the reduction of existing coloration of a liquid ring-polyalkyl-substituted aromatic primary diamine by blending into such diamine a color-reducing amount of at least one dihydrocarbylhydroxylamine. Preferably, the resultant product is stored in a closed container under an inert atmosphere.
U.S. Pat. No. 6,127,505 discloses an optically clear, high hardness, impact resistant polyurethane which is said to provide exceptionally high heat distortion temperatures and excellent chemical resistance. The invention is also said to be particularly useful for transparency applications that require excellent impact resistance coupled with high heat distortion temperatures.
U.S. Pat. No. 6,174,984 discloses a polyurethane elastomer that comprises the reaction product of: A) a prepolymer of at least one diisocyanate and at least one polyether polyol selected from the group consisting of a homopolymer of ethylene oxide, a homopolymer of propylene oxide, and a copolymer of ethylene oxide and propylene oxide, wherein free diisocyanate has been reduced to a level of less than 1% of the prepolymer; B) at least one alkylated aromatic diamine in a quantity sufficient to react with about 50% to about 105% of the available isocyanate content in the prepolymer; and C) at least one catalyst in a quantity sufficient to reduce the pot life to no more than two minutes, whereby said elastomer possesses high resilience and clarity and can be processed safely and at low cost.
Japanese Patent No. 03195770 discloses a blend comprising (a) an organic polyisocyanate, (b) an organic polyisocyanate reactive polymer with a molecular weight of 800-12,000, (c) 5-50 wt. % per (b) of a chain extender containing a hindered aromatic diamine, (d) 0.01-2.0 wt. % of a steric hindered amine type light stabiliser per (b) and optionally a stabiliser for urethane so that the blend has an isocyanate index of 70-130. The blend is treated in a one shot system by reaction injection moulding to make a shaped article of urethane/urea and polyurea. The shaped article is said to have improved paintability, e.g., fading resistance and anti-yellowing property.
Component (d) is preferably at least one of bis(2,2,6,6-tetramethyl-4-piperazyl)sebacate, bis(1,2,2,6,6-pentamethyl-4)sebacate, 1-(2-(3-(3,5-di-t-butyl-4-hydroxyphenyl)propionyloxy))-2,2,6,6-tetramethylpiperizine, 8-benzyl-7, 7,9,9-tetramethyl-3-octyl-1,3,3-triazaspiro(4,5)undecan-2,4-dione, 4-benzoyloxy-2,2,6,6-tetramethylpiperazine and 8-acetyl-7,7,9,9-tetramethyl-3-dodecyl-1,3,8-triazaspiro(4,5)undecan-2,4-dione. Component (c) is 2,4-diaminomethylene, 1,3,5-triethyl-2,4-diaminobenzene, 1,3,5-triisopropyl-2,4-diaminobenzene, 1-methyl-3,5-diethyl-2,4-diaminobenzene or 1-methyl-3,5-diethyl-2,6-diaminobenzene etc. Component (a) is a modified polyisocyanate containing 4-4xe2x80x2-diphenylmethanediisocyanate or the isomer.
It has been found that incorporating a small amount of a hindered amine light stabilizer (HALS) into a high melting aromatic diamine, such as MOCA, can effectively reduce the discoloration of the aromatic diamine at high temperature. Reduction in color formation with just 0.1% of Tinuvin 765 (Ciba-Geigy) added can improve discoloration by 300-400%. Furthermore, the assay of the aged aromatic diamine is greatly improved by the addition of the HALS.
More particularly, the present invention is directed to a method for stabilizing aromatic diamines at high temperature, which comprises blending in at least one HALS into the aromatic diamine before aging.
In another aspect, the present invention is directed to a composition comprising a blend of an aromatic diamine and a stabilizing quantity of a HALS, whereby the high temperature stability aromatic diamine is improved.
In still another aspect, the present invention is directed to an improved method for polyurethane elastomer production, wherein the aromatic diamine curative has a stabilizing quantity of HALS blended in, thereby improving the stability of the curative, and the color and quality of the polyurethane elastomer products made.
Hindered amine light stabilizers (HALS) are well known in the art. Tinuvin 765 (Ciba Geigy) is a (1,2,2,6,6-pentamethyl-4-piperidinyl)sebacate mixture. While there are a number of different HALS available on the market today, they are similar in chemistry and function, and tend to differ primarily in physical attributes, such as melting point, volatility, and concentration of active species.
HALS are typically used to prevent discoloration of polymers when exposed to light. They are generally not used to stabilize aromatic amines or other monomeric materials. Furthermore, they are not considered very effective against high temperature degradation. This function is left to primary antioxidants, for example, hindered phenolics and amines, such as Naugard 10 (Crompton Corp.) and Naugard 445 (Crompton Corp.), respectively.
Surprisingly, however, the primary antioxidants tested for the stabilization of MOCA did not prevent discoloration, whereas the HALS did. The quantity of HALS required is 0.01% to 5%, and preferably 0.02% to 1.0%.
Other stabilizers well known in the art include phosphite peroxide decomposers (such as Naugard P, Crompton Corp.), thioester antioxidants, such as pentaerythritol tetrakis(3-dodecylthiopropionate) (Seenox 412S), dialkyl hydroxylamine antioxidants (such as Irgastab FS042, Ciba Geigy), benzophenone UV stabilizers (such as Cyanox 2246, Cytec), and benzotriazole UV absorbers (such as Tinuvin 213, Ciba Geigy). Results of testing with these stabilizers are shown in the examples that follow.